This invention relates to an ultrasonic instrument for measuring the mass transport rate of solids suspended in a gas. More specifically, this invention relates to the transport of finely divided solids suspended in a gas in which an ultrasonic signal is used to measure both the solids loading in the gas and the average gas velocity to thereby determine the mass transport rate of both solids and gas.
In coal-fired steam generators of the type typically used in the electric utility industry, coal is pulverized in mills and then transported pneumatically via heated primary air through burner feed lines to a number of burners within the steam generator henceforth referred to as a "boiler". The coal pulverizing mills vary in size, but for the larger boiler installations today, typical operation might involve a feed rate of about fifty tons per hour of coal and one hundred tons per hour of primary air to each mill. The feed coal (usually in lumps about one-half inch in diameter) is typically pulverized to about eighty percent through 200 mesh (74 micron) screen and then the coal-air mixture is blown out of the mill into a number of sixteen-inch diameter pipes which feed burners on the sides of the boiler. A large boiler might have seven such mills each feeding eight burner feed lines, thereby feeding a total of fifty-six burner feed lines.
A significant problem for large-scale coal fired boiler operation arises from non-uniform distribution of the pulverized coal into the burner feed lines. In the above example, one-eight of the output from each coal mill should go into each of the feed lines. Presently, however, no reliable way to determine the actual coal distribution among the various burner feed lines exists. If a burner feed line transports too little coal causing the burner to burn lean, the boiler efficiency is degraded. If a burner feed line has too much coal causing the burner to burn rich, corrosion and fouling of the internal water tubes can occur which can reduce boiler efficiency and lead to costly shutdowns. In present practice, the primary air flow rate (without coal) is balanced with standard pitot-tube type flow meters which are removed before coal is added to the mill. Unfortunately, this method is inaccurate since it does not directly measure coal loading, and further, it suffers the disadvantage that continuous readings during boiler operation are not available.
In view of the foregoing, a reliable on-line instrument is certainly needed for measuring coal loading and mass flow rates in boiler feed lines. Such an instrument could be used to provide an indication of coal loading to enable an operator to take appropriate steps or could be used to directly control coal loading. Development of such an instrument, however, poses serious design challenges due to the harsh flow conditions in burner feed lines since the gas velocity (approximately 30 m/sec.) must be high to prevent the boiler flame from flashing back into the burner feed lines and the coal loading (approximately 0.5 kg coal/kg gas) makes a very abrasive medium in the highly turbulent gas stream.
Over the years, many instruments based on various physical phenomenon have been developed to measure fluid velocities in both single and two-phase flow. In general, those instruments that have been successful have been used in relatively mild fluid environments as compared to the flow conditions for which the present invention is directed. The British have been working on the problem of measuring flow in boiler burner feed lines for about fifteen years. In one system that was developed and tested at a power plant site, the Doppler shift phenomena of an ultrasonic signal was used to determine the gas velocity in a burner feed line and the attenuation of a nuclear generated beam of Beta particles was used to determine the coal loading. A test of the instrument at the power plant site confirmed that the distribution of coal among the burner feed lines from the mill is, in fact, non-uniform. Unfortunately, that instrument package was judged to be not sufficiently reliable for commercial application.
The present inventors turned their attention toward the design of an instrument for measuring the mass flow rate of finely divided coal using ultrasonic waves to determine both gas velocity and solids loading. One of the characteristics of the high-velocity gas flow in a burner feed line in which a mass flow meter must operate that has made the development of an instrument so difficult in the past is the highly turbulent field generated at the high Reynold's Numbers (Re=500,000) which are found in burner feed lines. This characteristic also makes the task of measuring the received ultrasound signals difficult because the sound is scattered by the eddies in the turbulent flow field. Not only do the turbulent eddies attenuate the strength of the sound arriving at the receiving transducer, but it also produces a highly fluctuating signal. Neither of these features facilitate the measurement of either the gas velocity or the solids loading.
Available Energy, Inc. and Detroit Edison Company of Detroit, Michigan, sponsored a study at Wayne State University from 1979 to 1985 of the physical phenomena present in a utility burner feed line. A mock-up of a full-scale (twelve-inch diameter) burner feed line was constructed in the shape of a sixty-seven foot long closed loop. Using mostly commercially available electronic instruments, ultrasonic signals were sent across the flowing gas stream to measure its interaction with the turbulence generated in the pipe and with suspended coal particles. This work led to a published Ph.D. thesis of Dr. Thomas A. Hamade of Wayne State University in 1982 entitled, "Ultrasonic Attenuation in Pipe Flow of Turbulent Gas and Suspended Particles", which is hereby incorporated by reference and will be referred to henceforth as the "WSU-Thesis". The WSU-Thesis work demonstrated that an ultrasound signal could be sent across a large coal-laden gas stream and that the interaction of the sonic waves with the gas turbulence and with the suspended coal particles could be reasonably predicted from existing physical theory. This work also demonstrated that the average gas velocity could be obtained from the expected and well-known downstream drift of the sound signal. The full-scale experimental results were new and contributed to the understanding of turbulence and the interaction of sound with the turbulence and suspended coal particles. However, the transducer mounts, the off-the-shelf electronics, the methods of operating the transducers, and the methods of processing the received signal in combination did not constitute an "instrument" that would produce a readout of either the gas velocity or coal loading. The present invention provides the means to automatically read out on-line the gas velocity and/or coal loading from the interaction of ultrasound waves and the flowing medium, thus providing an instrument suitable for commercial use. An alternate embodiment of an instrument according to this invention would further provide a measurement of the temperature of gas within a conduit by measuring the rate of propagation of an ultrasonic wave through the gas.
In the patent of the present continuation-in-part application, an ultrasonic mass flow meter was described in which a fixed transmitting transducer was used to send a pulsed beam of ultrasound across the diameter of the burner feed pipe. Due to the flow of gases and solids within the conduit, the packet of ultrasound energy becomes displaced downstream as it traverses the pipe. A measure of velocity was obtained by incrementally moving the receiving transducer over a range of longitudinal downstream positions to determine when the received signal was at its highest intensity. This downstream drift is then equated with gas flow velocity. Although this configuration for the transducers operates satisfactorily, it has one very significant disadvantage. Since it is necessary to move the receiving transducer downstream, the burner feed lines must be cut in order to install the transmitting transducer and sliding receiving transducer mount. Since many of utility power plant feed lines are schedule 40, 16-inch diameter pipe, it is very expensive and time consuming to cut such burner pipes. Accordingly, it is desirable to provide a transducer arrangement which simplifies installation for existing burner feed lines.
In accordance with the present invention, two embodiments for improved transducer configurations are disclosed. In a first embodiment referred to as a "variable-pitch" embodiment, a pair of transducers are mounted in ball-and-socket type mounts positioned diametrically opposite each other. The transmitting transducer projects an ultrasound pulse in an upstream direction which drifts downstream with respect to the fluid stream to be captured by the receiving transducer at the diametrically opposite position. The receiving transducer is angled to properly intercept the received wave packet. For this embodiment, only the angles and not the linear (downstream) positioning of the transducers is varied as a means of measuring downsteam drift of the ultrasound packets.
In another embodiment of this invention, termed a "phased-array" embodiment, a pair of transducers are provided positioned diametrically opposite each other. Each transducer is comprised of an array of a number of individually actuated or sensed zones. By employing a phase difference in the energization of the discrete regions of the transmitting array, the resultant ultrasound beam can be aimed or steered upstream at a predetermined and variable angle. Outputs of the individual zones of the receiving transducer array are phase shifted to properly receive the wave packets. Like the first described embodiment, this design significantly reduces costs associated with adapting the mass flow meter of the present invention to existing power plant installations since simple pipe connections can be used to mount the transducers.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.